1. Field of the Invention
The present invention relates to a canister purging apparatus, which causes a canister to adsorb fuel evaporative gas generated in a fuel tank of an internal combustion engine and to purge fuel evaporative gas during a prescribed engine operating state into a suction pipe. Furthermore, the present invention relates to an air-fuel ratio control system.
2. Related Art
Canisters have recently been designed so as to have larger capacities, which respond to evapo-emission and OVR regulations enacted in the United States. Quick purging of a fuel-evaporative gas, which is adsorbed in large quantities in a large-capacity canister, is thus required. Under these circumstances, conventional techniques have been proposed for improving the ability to control the air-fuel ratio by correcting the quantity of injected fuel in response to the concentration of evaporative fuel during purging and by controlling the purging flow rate. For example, Japanese Patent Provisional Publication No. 63-289,243 discloses such a technique.
However, the method and apparatus disclosed in such a document only allow control of a purge valve in a fully open or fully closed state. The fully open state is thus maintained at a small value so as to ensure that an enriched air condition never occurs, even if the purge valve is fully opened and the fuel evaporative gas is at its highest concentration. In some states where purging large quantities of fuel evaporative gas is desirable, such as during high-load operation, it is impossible to increase the amount of purging. Because the purge valve can be controlled only at a fully open or fully closed state, purging cannot be accomplished when the operating state is one where using a fully-opened purge valve results in an enriched condition. Thus, even if slight purging might be possible, the system is unable to accomplish purging. Even when fuel evaporative gas is generated in large quantities, such as when the engine is run in the summer, it is impossible to increase the amount that the engine can purge or increase the operating condition that allows purging. This results in a canister that suffers from too great a load and increases the risk of fuel evaporative gas being discharged in large quantities.
A system is also available that corrects the purge flow rate by relying upon duty control. In this system, however, air intake quantity and the number of engine revolutions determine the purge quantity. The purge quantity is controlled irrespective of how much fuel evaporative gas is adsorbed into the canister. This system is thus limited in its use, in that purging is only permitted in an operating condition such that the extent of fuel concentration being purged is not important.
Conventionally there have been systems that permit purging for only a limited amount of operations, or systems that permit purging in a limited quantity.
A conventional air-fuel ratio control system requires that a feedback correction factor, including both an air-fuel ratio correction factor and a learning value, be monitored to reduce the difference in the air-fuel ratio between an actual ratio observed with an air-fuel ratio sensor and a target ratio. Feedback control and learning control of the air-fuel ratio are performed by means of this air-fuel ratio correcting factor (feedback correcting factor and learning value). The quantity of fuel supplied to the internal combustion engine is changed in response to the quantity of discharge (purged) evaporative gas, thus determining the quantity of injected fuel for the injector. Such a system is, for example, disclosed in Japanese Patent Provisional Publications Nos. 63-186,955 and 2-130,240.
More specifically, in such an air-fuel ratio control system, an evaporative gas purge correction factor is set so as to be proportional to the quantity of discharged evaporative gas. The quantity of supplied fuel based on the evaporative gas is estimated by means of multiplying the evaporative gas purge correction factor and the quantity of fuel supplied to the internal combustion engine. Such a product is called the corrected decrement quantity. By subtracting this quantity of supplied fuel in the form of evaporative gas from the quantity of fuel supplied to the internal combustion engine, a quantity of injected fuel for the injector may be calculated. The evaporative gas purge correction factor indicates the ratio of the quantity of the fuel supplied in the form of evaporative gas relative to the quantity of supplied fuel to the internal combustion engine during operation of the engine.
A problem found in conventional air-fuel ratio control system, such as those described above, is that, when the constant correction factor including the air-fuel ratio correction factor is different than 1.0, the quantity of injected fuel from the injector deviates from the desired control target value. This exerts an adverse effect on the control of the air-fuel ratio.
Furthermore, another conventional air-fuel ratio control system for internal combustion engines is disclosed in Japanese Patent Provisional Publication No. 63-255,559. This system is based on a method of gradually changing a purge ratio, which has been previously set in compliance with operational conditions of an internal combustion engine (engine revolutions, quantity of air intake, etc.) until a prescribed value is reached, thus reducing the difference between the air-fuel ratio of the mixture of gas or fuel supplied to the internal combustion engine and air. The target air-fuel ratio is also reduced, caused by the delay in detection response.
When the purge ratio is very low in the air-fuel ratio control of an internal combustion engine, or when, in spite of a rather high purge ratio, the absolute flow rate of air intake upon idling is very low, it is possible that the duty ratio for pulsation-driving of a VSV (Vacuum Switching Valve), which is a flow control valve for purging, drops. When this duty ratio decreases to under 15% as shown in FIG. 2, pressure pulsations in the intake manifold and variation of the purge flow rate result in considerable fluctuations of the air-fuel ratio. Hence, such fluctuations result in a deteriorated discharge emission.